Such methods for determining and monitoring fill level in a container are often used in measuring devices in the fields of automation- and process control technology. For example, measuring devices under the names Micropilot or Levelflex are produced and sold by the assignee. These measuring devices function according to the travel-time measurement method, and serve for determining and/or monitoring a fill level of a medium in a container. In the guided microwave method, or the TDR (Time Domain Reflection) method, a high-frequency pulse is emitted along a Sommerfeld or Goubau wave guide, or along a coaxial wave guide. This pulse, upon meeting a discontinuity in the DC-value (dielectric constant) of the medium surrounding the wave guide, is partially reflected back. In the freely-radiating, travel time measuring method, for example, microwaves are emitted via an antenna into a free space, and the echo waves reflected on the surface of the medium are then received back by the antenna following the separation-dependent, travel time of the signal. On the basis of the time lapse between the transmission of the high-frequency pulse and the reception of the reflected echo signals, the separation from the measuring device to the surface of the medium can be obtained. Taking into account the geometry of the interior of the container, the fill level is then determined as a relative or absolute parameter.
The travel time measuring method can be divided essentially into two categories: The first is based on a time measurement and requires a pulse sequence-modulated signal for the distance covered; a second widely-used evaluation method is to determine the sweep difference frequency between the emitted, continuous, high-frequency signal and the reflected, received high-frequency signal (FMCW—frequency modulated continuous wave). In general, the following explanations are not limited to a specific evaluation method.
A general problem in the case of all travel time measuring methods using high-frequency measuring signals in the GHz (Gigahertz) region is that, for the evaluation of high-frequency measuring signals, high-frequency components, which are designed for such high-frequency ranges, must be used. These high-frequency components have the disadvantage that they are complex to produce and very expensive to purchase. One possibility for evaluating the high-frequency measuring signals using inexpensive low-frequency components is to map the high-frequency signals into the low-frequency range by means of a sequential sampling. The method for the sequential sampling of high-frequency measuring signals represents a possibility for transformation into the low-frequency range, wherein, in this method, a time-expanded, intermediate-frequency signal is generated from a multiplicity of quasi high-frequency, periodically-sampled, measurement signals. This additional processing step is carried out because there are no suitable, cost-effective data-processing units, e.g. DSPs (Digital Signal Processors) which can reliably process high-frequency measuring signals.
An approach for generating a time-expanded, intermediate-frequency signal is the mixing principle in which two oscillators produce two oscillations with slightly different frequencies. Through the slight “detuning” of the frequencies of the two oscillations, a phase shift results, which increases linearly with each measuring period. This phase shift corresponds to a linearly-increasing time delay.
The mixing principle is illustrated for example in DE 31 07 444 A1 by means of a high-resolution, pulse radar method. A generator produces first microwave pulses and radiates them via an antenna, at a predetermined transmission repetition frequency, in the direction of the surface of the filling substance. A further generator produces reference microwave pulses, which correspond to the first microwave pulses, but differ slightly from these as regards the repetition frequency. The echo signal and the reference signal are mixed, for example, by a frequency converter, or mixer, whereby an intermediate frequency signal is generated. The intermediate frequency signal has the same behavior as the echo signal, but is, however, stretched relative thereto by a transformation factor, which is equal to a quotient of the pulse repetition frequency and the difference frequency between the pulse repetition frequency of the first microwave pulses and the sampling frequency of the reference microwave pulses.
In the case of a pulse repetition frequency of several megahertz, a difference frequency of a few hertz, and a microwave frequency of several gigahertz, the frequency of the intermediate frequency signal lies well below 200 kHz. The advantage of the transformation to the intermediate frequency is that relatively slow, and thus inexpensive electronic components for registering and/or evaluating signals can be used.
Referenced in this connection is also German Utility Model DE 29815069 U1, which describes this known transformation technology in the case of a TDR-fill level measuring device. This sampling circuit has two oscillators, at least one of which has a variable frequency, with one oscillator driving the transmitting generator, and the other driving the sampling pulse generator. A frequency mixer forms the difference of the two frequencies, which is used for adjusting, or controlling, the transformation factor, which is to be as constant as possible, to a desired value via a feedback branch.
A disadvantage of regulating the difference frequency to a desired value according the state of the art is that such a control takes a very long time, and, under certain conditions, results in regulating to an incorrect desired value of the difference frequency.